The present invention relates to a light emitting diode device and, more particularly, to a light emitting diode device that produces white light by combining primary bluish-green light with phosphor-converted reddish light to produce white light.
Light emitting diode devices (LEDs) that are utilized in green traffic lights emit a bluish-green light at wavelengths in the range of approximately 485 to 515 nanometers (nm). The wavelength emission range is specified by law and is a very narrow subset of this range. When these LEDs are produced, they are tested to determine whether or not the emitted light is within the legally specified range. If the LEDs do not meet these specifications, they typically are discarded.
With the development of efficient LEDs that emit light in the spectral range of 450 to 520 nanometers (nm), it has become feasible to produce LEDs that generate white light through phosphor conversion of a portion of the primary emission of the LED to longer wavelengths. An unconverted portion of the primary emission combines with the light of the longer wavelength to produce white light. In order for an LED to produce white light that is good for illumination purposes (i.e., that has good color rendering capabilities), the primary emission should be at a wavelength of less than, for example, 480 nm. However, if the LED is to be used only for signaling or display purposes, the primary emission can be at a wavelength of greater than 480 nm. Phosphor conversion of primary light at a wavelength of greater than 480 nm will still produce light that appears white when viewed directly or through a diffusion screen on which the light impinges. Such an LED would be suitable for use in various types of displays and signaling environments (e.g., pedestrian traffic lights), although it generally would not be suitable for illumination purposes due to its poor color rendering characteristics.
Pedestrian traffic lights are currently the only types of traffic lights that do not utilize LEDs. Pedestrian traffic lights generally use incandescent bulbs for signaling. These bulbs burn out relatively quickly and must be replaced relatively often. It would be desirable to provide an LED that produces white light and that is suitable for this purpose and for other signaling and/or display purposes. LEDs consume less energy than incandescent bulbs and normally have lifetimes that are much greater than incandescent bulbs. Consequently, LEDs require less maintenance and can potentially provide a more economic solution than incandescent bulbs in these types of environments.
Accordingly, a need exists for a white-light emitting LED that is suitable for signaling and/or display purposes.
The present invention provides a white-light emitting diode (LED) that emits primary light at a wavelength that is in the range of approximately 485 to approximately 515 nm, which corresponds to a color of bluish-green. A portion of the primary light is converted into a reddish-colored light that ranges in wavelength from approximately 600 to approximately 620 nm. At least a portion of the converted light combines with the unconverted portion of the primary light to produce white light. The LED comprises a substrate and a light-emitting structure, which is disposed on a surface of the substrate. The light-emitting structure emits the primary light that impinges on a phosphor-converting element to produce the reddish-colored light that combines with bluish-green light to produce white light.
A number of phosphor-converting elements are suitable for use with the LED of the present invention, including a resin admixed with a phosphor powder, epoxies admixed with a phosphor powder, organic luminescent dyes, phosphor-converting thin films and phosphor-converting substrates. Preferably, the phosphor-converting element is a resin admixed with a phosphor powder in such a manner that a portion of the primary light impinging on the resin is converted into the reddish-colored light and a portion of the primary light passes through the resin without being converted. The unconverted primary light and the phosphor-converted reddish-colored light combine to produce white light. In accordance with this embodiment, the LED is mounted in a reflector cup that is filled with the phosphor-converting resin. The LED can be mounted in either a normal or flip-chip configuration within the reflector cup.
Other features and advantages of the present invention and variations thereof will become apparent from the following description, drawings and claims.